1. Field
An absolute angle detecting device is provided.
2. Related Art
An absolute angle detecting device is provided between a steering shaft and a body of an automobile. Generally, the technology in which the control of the damping force of a suspension, the shift position control of an automatic transmission, and the steering control of rear wheels in a four-wheeled motor vehicle, are performed on the basis of the rotating angle of the steering wheel, the steering speed, the steering direction, or the like, detected by means of the absolute angle detecting device, is known. The steering shaft is a multi-rotation body configured to rotate to the right or left direction for two to three rotations from a neutral position. Therefore, an absolute angle detecting device that is capable of detecting the angle of one or more rotations of the steering shaft is required.
An absolute angle detecting device detects the absolute angle at a certain resolution. The device uses a BCD code, an M-system code or a Gray code, for example. It is common for the absolute angle detecting device to use the Gray code. This is because only one bit varies constantly in each of a leading step and a trailing step in the concatenating Gray code and thereby timing pulses for reading are not required and it is beneficial for achieving high resolution (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-28396).
In the absolute angle detecting device for use in the automobile, it is common that a rotatable angle of the steering shaft (for example, ±720 degrees through ±1080 degrees) is divided into a certain angle area called the “sector.” A Gray code is assigned in the sector at a certain resolution. There are some types of the absolute angle detecting device that identify each of the sectors, such as the rotation of the rotary disk that is connected to a steering shaft via a reduction gear mechanism such as a train of gears is detected by means of detecting element. A convolution-shaped groove is concentric with a rotation center of a rotary disk connected to a steering shaft is formed. An actuator of a linear position detecting device or a rotary position detecting device is inserted into the convolution-shaped groove. The rotation of the rotary disk is converted into an amount of displacement, so that the sector is detected up to one or more rotations of the steering shaft.
Since it is impossible to avoid a backlash or the like from the mechanical speed reduction mechanism, such as a train of gears, the combination of the convolution-shaped groove and the actuator, or the like, the deviation occurs at the timing for switching a digital code that detects an angle in a sector and a digital code for indicating the sector, in the sector identification device having such a mechanical speed reduction mechanism.
The deviation amount a is found by the equation: α=±m·n, where a detecting error of the sector due to the backlash or the like is defined as m degrees and a speed reduction ratio of the speed reduction mechanism is defined as n. As represented by the equation, the rotating angle of the multi-rotation body cannot be correctly detected because, the greater the speed reduction ratio becomes, for example, the greater the detecting angle area becomes, the greater the deviation amount α becomes.